<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="https://unpkg.com/vue"></script>
</head>
<body>
<div id="demo">{{fullName}}</div>
<script>
  var vm=new Vue({
      el:'#demo',
      data:{
          firstName:'Foo',
          lastName:'Bar',
          fullName:'Foo Bar'
      },
      watch:{
          firstName:function(val){
              this.fullName= val +'' +this.lastName
          },
          lastName:function(val){
              this.fullName=this.firstName+''+val
          }
      }

  })

</script>
</body>
</html>